The present invention relates to a brush-less motor having in a rotor one magnet whose multi-poles are magnetized and an electric power steering device for applying a steering assist force to a steering mechanism of a vehicle by the brush-less motor.
An electric power steering device has been hitherto used that an electric motor such as a brush-less motor is driven in accordance with a steering torque applied to a handle (a steering wheel) by a driver.
The brush-less motor ordinarily used in such an electric power steering device is a permanent magnet type electric motor including a stator having a winding (coil) and a rotor provided with a permanent magnet, and most of the brush-less motors are driven by a three-phase electric power. As for the windings of the stator, plural windings are ordinarily provided for each phase and a plurality of poles of the permanent magnet are also ordinarily provided correspondingly thereto. Accordingly, the permanent magnet preferably has a radial anisotropy in which the multiple poles can be easily magnetized.
In fact, since the magnet having the radial anisotropy is hardly inexpensively produced, a magnet having a polar anisotropy that is relatively inexpensive and high in its magnetic flux density may be sometimes used. Since the magnet having the polar anisotropy is magnetized according to a magnetic path given during a molding process, the poles cannot be arranged at arbitrary positions as in the magnet having the radial anisotropy. Accordingly, when what is called a skew is tried to be applied (namely, skew-magnetized) to the magnet having the polar anisotropy to be used as a rotor, it is impossible to arrange the poles obliquely with respect to the axial direction on the surface of one magnet having the polar anisotropy. Thus, in place of such a structure, for instance, there is a structure that two magnets having the polar anisotropy are prepared and these magnets are shifted from each other by an angle of skew on an axis of a cylinder and axially connected (see JP-A-2001-314050). A motor including the rotor composed of the magnet having such a usual structure can reduce or cancel at least a low order cogging.
To sufficiently reduce or cancel the cogging including a high order cogging, the skew is ordinarily applied so that the poles are respectively arranged obliquely with respect to the axial direction on the surface of the magnet as the rotor as described above. The skew is ordinarily obliquely magnetized, as mentioned above, on the above-described magnet having the radial anisotropy.
When even the magnet having the radial anisotropy has a special shape, a structure may be sometimes used that two magnets are shifted from each other around an axis of a cylinder and connected in the axial direction (see JP-A-2005-65455). In this usual structure, the cylindrical magnet having the radial anisotropy has a special shape that a plurality of cut-out parts are provided along the axial direction on a surface thereof. Accordingly, in this usual structure, a magnetization is simply carried out along the axial direction so as to meet the special shape. As long as the magnet does not have such a special shape, to sufficiently reduce or cancel the cogging, a skew is ordinarily applied to the magnet having the radial anisotropy so that the poles are respectively obliquely arranged.
However, the motor having the ordinary and usual magnet having the radial anisotropy in which the skew magnetization is obliquely carried out as described in JP-A-2001-314050 cannot adequately reduce a cogging generated owing to the shape of a stator.
For instance, the stator provided so as to be opposed to the rotor preferably has the same shape in any of sections perpendicular to the axial direction of the rotor correspondingly to the cylindrical shape of the rotor. However, typically, when coil ends for fixing the winding at opposite ends of the stator are provided, parts in the vicinity of the opposite ends of the stator may not possibly have the same sectional shapes along the axial direction due to the three-dimensional shapes of the coil ends. According to such a structure, in parts near the opposite ends of the rotor corresponding to parts near the coil ends, a cogging including irregularly varying components corresponding to the three-dimensional shapes arises.
In a structure with such a shape of the stator in which the skew magnetization in the oblique direction is carried out, since a cogging of an opposite phase to cancel the above-described cogging including the irregularly varying components with each other does not include the irregularly varying components, these cogging does not completely cancel with each other. After all, the cogging cannot be sufficiently reduced.
Namely, in the above-described structure in which the skew magnetization in the oblique direction is carried out, the lowest order cogging generated in a certain position of the rotor has an opposite phase to a phase of a cogging generated at a position spaced by a distance substantially half as long as a length in the axial direction from that position. Accordingly, the cogging whose phase is opposite to the phase of the cogging generated in the parts in the vicinity of the opposite ends of the rotor corresponding to the parts in the vicinity of the coil ends of the stator arises in a part in the vicinity of the center of the rotor. Then, since the cogging arising in the vicinity of the center of the rotor does not include, as a matter of course, the irregularly varying components, the cogging cannot be finally sufficiently reduced. This will be briefly described by referring to FIGS. 5 and 6.
FIG. 5 is an external appearance view simply showing teeth and a magnetized state of the usual permanent magnet. FIG. 6 is a wave form diagram for explaining that the cogging is not reduced or cancelled by the usual structure. Referring to FIG. 6, the cogging generated at a position A and a position D includes the irregularly varying components due to the coil ends having the three-dimensional shapes as shown in FIG. 5. However, since the cogging does not have the opposite phases, they cannot be cancelled with each other. As a result, the cogging is not adequately reduced.